This invention relates to an apparatus and method for controlling a vehicle having a prime mover and a secondary mover which incorporates an energy storage device. The invention has particular application to a hybrid vehicle whose prime mover is an internal combustion engine and whose secondary mover incorporates a flywheel but is not limited to this combination.
SAE technical paper 2008-01-0083, Apr. 14-17, 2008, describes an arrangement consisting of a continuously variable transmission (CVT) connected between the engine and gearbox of a vehicle and configured to drive a flywheel through a gearset. The arrangement can add or subtract power to that supplied by the engine to a driven pair of wheels of the vehicle. The flywheel can be spun up in order to capture the kinetic energy of the vehicle as it decelerates. This is regenerative braking.
U.S. 2008/0105475 discloses an automotive vehicle having a powertrain for the vehicle, a flywheel and a continuously variable transmission which interconnects the powertrain and the flywheel. When the vehicle decelerates, energy from the powertrain and its associated transmission is transferred to the flywheel. When the vehicle accelerates, the energy of the flywheel is transferred to the powertrain.
In a flywheel-based energy storage and recovery system, manipulation of the CVT ratio achieves control of energy storage and recovery. When the ratio is set so as to speed up the flywheel, energy is stored and when the ratio is set so as to slow down the flywheel, energy is recovered.
The power output from an energy storage device such as a flywheel can supplement that of the prime mover. Consequently it is possible to increase the total output power of the vehicle or downsize the prime mover. The latter option provides further fuel efficiency savings. However, particularly in the case where the prime mover is an internal combustion engine, a downsized engine produces correspondingly lower levels of low speed torque compared with a conventionally-sized engine. This is likely to cause “drivability” problems. For example, when pulling away from rest and when the flywheel has low or zero levels of stored energy, the secondary mover is unable to supplement the power available from the engine and there is a deterioration in the vehicle's performance as perceived by the driver. Hence there can be an inconsistency of supplementary power available for a hybrid vehicle. This can cause drivability concerns and preclude opportunities for engine downsizing.
The present invention aims to mitigate this deleterious effect on drivability.